Aetna, the nation’s third largest health insurer, announced earlier this month that it is sharply reducing its participation in the marketplaces set up by the Affordable Care Act, raising doubts about how the nation’s health care law is functioning six years after its enactment.

Aetna’s decision to cut its participation from 15 state marketplaces to four comes as other insurers confront losses and increased risk in states that have smaller populations and lower participation in the public exchanges.

“As a strong supporter of public exchanges as a means to meet the needs of the uninsured, we regret having to make this decision,” said Aetna Chairman and CEO Mark Bertolini. “Providing affordable, high-quality health care options to consumers is not possible without a balanced risk pool.”

Aetna said it is encouraged by the Administration’s recent commitment to explore new options to modify the risk adjustment program and hopes to work with policymakers on other exchange-related policy improvements that could allow the company to expand its footprint in the future.

Obamacare is still succeeding at increasing the number of Americans who have health insurance, but the law is failing to deliver on its promise of bringing competition to the individual marketplace, according to a recent piece by Vox senior editor Sarah Kliff.

The reduced competition in many states is likely to drive up the cost of premiums in 2017, health care experts say. Insurers are seeking approval for average national premium increases of 24 percent, according to calculations posted on ACAsignups.net.